1. Field
The present invention relates to a device which detects a magnetic pole position of a permanent magnet motor having a magnetic saliency.
2. Related Art
Recent washing machines and the like have employed an arrangement of vector control for a permanent magnet motor for the purpose of improving a precision in rotational control or washing performance. As a result, power consumption and an amount of vibration produced during the operation have been reduced in the washing machines and the like. Electrical current is controlled according to a position of magnetic pole of a permanent magnet motor when the vector control is applied to the motor for control in high precision and high rotational speed. In this case, a position sensor is necessitated for this control manner. However, addition of a position sensor results in a problem of increasing production costs, a problem that an arrangement space needs to be ensured for the position sensor, and a problem of increase in the wiring for connecting between the position sensor and a control device. Furthermore, there arises a problem of reduction in the reliability due to possible occurrence of breaking or disconnection or a problem of maintenance.
In view of the above-described problems, there has been provided a sensorless drive system in which an induced voltage in a permanent magnet is detected and a magnetic pole position using the relationship between the induced voltage and a magnetic pole position but without any position sensor. However, there is a problem that the position detection cannot be executed in a stopped state where no induced voltage is generated or in a very low speed range in which voltage amplitude is too small.
On the other hand, there is a system that executes a detection of magnetic pole position using a magnetic saliency regarding an embedded permanent magnet having a magnetic saliency or a reluctance motor. Since inductance is distributed according to a magnetic pole position in these motors, a high-frequency current or voltage is applied to the motor so that a motor current and voltage are detected. Inductance is then obtained from the applied high-frequency current or voltage, so that a magnetic pole position is obtained (see Japanese patent application publication JP-A-2001-339999).
However, the inductance distribution of each aforesaid motor appears according to the magnetic salience. Accordingly, a clear distribution is obtained from a motor having a larger salience ratio (Lq/Ld) but in the motor having a smaller salience ratio, an inductance distribution is buried in an electrical or mechanical error of the motor or calculating error in a calculating unit, whereupon there is a problem that the position detection becomes difficult. Furthermore, the motor causes magnetic saturation when a relatively larger current is supplied to the motor. Even when a q-axis inductance Lq is saturated, the salience ratio becomes smaller as compared with the case where no power is supplied to the motor. As a result, too, the position detection becomes difficult.